Leeches have a wonderful talent for making human skin crawl. Soft, blind and voracious, they fasten on, slice the skin and drink until they are fat with blood.
Most of the time, though, the real horror is psychological rather than mortal. A leech on your calf is more likely to leave you with an oozing bite and a strong desire never to paddle barefoot again than to kill you. But in rare circumstances, leeches can become genuinely dangerous.
- It's as big as a rolling pin and can drain a tablespoon of human blood in a single meal, thanks to its razor-sharp hypodermic needle-like mouth
- Leeches can jump, say scientists. Don't believe them? Watching this video will make you shudder...
What is a leech, and how does it drink blood?
A leech is a segmented worm, related to earthworms, belonging to the subclass Hirudinea. There are hundreds of species, and not all of them feed on blood. Some hunt small invertebrates and some eat organic debris, while others parasitise fish, amphibians, birds or mammals.
Blood is a rich food source, packed with protein and other nutrients. For blood-feeding leeches, it offers a way to take in a large meal in one go. Some species can drink up to 10 times their body weight during one feed, enabling them to survive for long periods before needing another meal.
This helps explain why leeches have evolved such extraordinary feeding equipment. A typical leech has a sucker at each end: one for gripping and moving, and one containing the mouth. In the European medicinal leech (Hirudo medicinalis), three jaws armed with tiny teeth slice into the skin, leaving a neat, Y-shaped bite.
Then the chemistry begins. Leech saliva contains substances that reduce pain, widen blood vessels and stop blood from clotting. One of the best known is hirudin, a powerful anticoagulant. This is why a leech bite can keep bleeding after the animal has detached.
What if you get lots of leech bites?
Multiple bites mean multiple open wounds and multiple little anticoagulant delivery systems working at once. Each individual bite may be minor, but together they can add up to significant blood loss, particularly in children, frail people or anyone already anaemic or unwell.
When do leeches become truly dangerous?
The worst cases usually involve internal leech infestation, known as hirudinosis. This can happen when someone drinks untreated water, or swims or bathes in water containing young aquatic leeches. Instead of attaching to the skin, the leech may fix itself inside the nose, throat, voice box or airway.
That is where the horror-film version becomes closer to reality. An internal leech can be hard to detect and may feed for days or weeks, while its anticoagulant saliva keeps the wound bleeding. Symptoms can include nosebleeds, coughing blood, hoarseness, difficulty swallowing, breathing problems or unexplained anaemia.
In the most serious cases, a leech can cause dangerous blood loss or obstruct the airway. Children are especially vulnerable because they have a smaller blood volume and narrower airways. Case reports describe severe anaemia caused by leeches in the mouth or throat, and medical literature records rare deaths.
So, can leeches kill you?
Very rarely, yes. One leech on the skin is usually disgusting rather than deadly. The real danger comes when leeches attach inside the body, cause prolonged bleeding or introduce infection. Their superpower is simple and sinister: they keep blood flowing. On a medical ward, that can help save a finger. In a throat, nose or airway, it can become a medical emergency.
